1. Field of the Invention
The present invention relates to a process for producing carbon fibers having modified surfaces, and more particularly to a process for producing carbon fibers having modified surfaces which are excellent in adhesion to matrix resins. The present invention further relates to carbon fibers having such modified surfaces.
2. Description of the Prior Art
Since composite materials using carbon fibers as reinforcement are light in weight and excellent in strength and elastic modulus, they are used in a variety of fields including parts for sports and leisure goods or materials for aerospace vehicles. However, since conventional carbon fibers used as reinforcement for composite materials are not necessarily satisfactory from the point of view of adhesion to the matrix resins, it is known to activate the surface by oxidizing with a chemical agent, in an oxidizing gaseous phase, or by electrolytic oxidizing treatment, thereby improving the adhesion of the carbon fibers to the matrix resins. Electrolyte oxidation is considered most practical from the viewpoint of its good operatability and ease reaction control.
In electrolytic oxidizing treatment, various electrolytes have been studied.
For example, U.S. Pat. No. 4,401,533 discloses a process wherein electrolytic oxidation is carried out using a carbon fiber as an anode in an aqueous sulfuric acid solution under the specified range of electric current, voltage and treating time.
U.S. Pat. No. 3,832,297 discloses that an ammonium compound is used as an electrolyte, electrolytic oxidation is carried out using a carbon fiber as an anode, and the compound decomposes at a temperature of lower than 250.degree. C. and does not remain on the fiber surface.
U.S. Pat. No. 4,867,852 discloses a process wherein after electrolytic oxidation is carried out by using an ammonium compound as an electrolyte and a carbon fiber as an anode, the carbon fiber is subjected to ultrasonic cleaning.
U.S. Pat. No. 4,600,572 discloses that when a carbon fiber is electrolytically oxidized in nitric acid and then subjected to an inactivation treatment, a carbon fiber having a high strength and excellent adhesion to resins can be produced.
Further, since sufficient surface treatment cannot be effected by the use of one electrolyte, performing of a two-stage electrolytic treatment is suggested in U.S. Pat. No. 4,839,006. However, in the prior technique, a satisfactory surface treatment effect cannot be obtained for high-modulus carbon fibers of a modulus of higher than 30 t/mm.sup.2.
U.S. Pat. Nos. 4,814,157, and 4,729,820 disclose processes wherein nitrogen functional groups are introduced onto the carbon fiber surface by a two-stage surface treatment.
Surface treatments other than oxidation, are known. For instance it is known to attach certain polymers to the surface of a carbon fiber by electrolytic polymerization as disclosed by R. V. Subramanian in Pure & Appl. Chem., Vol. 52, pp. 1929 to 1937 (1980).
Year by year, however, the demand for enhancing the performance of carbon fibers is increasing. In particular, the development of carbon fiber for aircraft has been directed to make high-strength and high-modulus carbon fiber, and recently, intermediate modulus carbon fiber having a modulus of about 30 t/mm.sup.2 is prevalent. On the other hand, the development of carbon fiber for the application to sports and leisure goods has also been directed to prepare high modulus carbon fiber having a modulus of about 45 t/mm.sup.2, and good composite properties has also been developed. However, as the modulus of the carbon fiber increases, the surface of carbon fiber becomes more inactive, and the interfacial bonding strength between the fiber and the matrix resin is reduced. Therefore, the conventional surface treatment techniques for carbon fiber are insufficient, and a surface treatment method of high-modulus carbon fiber has not yet been developed for making the composite performance, particularly ILSS (interlaminar shear strength), TS .perp. (transverse tensile strength), and FS .perp. (transverse flexural strength) satisfactory. One known surface treatment of carbon fibers involves introducing oxygen or nitrogen functional groups onto the surface, or attaching polymer to the surface of a carbon fiber by electrolytic polymerization. However, since it is considered that oxygen or nitrogen functional groups are introduced only on the edge of the graphite crystal on the surface of the carbon fiber, in the case of high-modulus carbon fiber wherein he graphite crystals are large, defects exist which is limit the introduction. Moreover, if the level of the electrolytic oxidation treatment is excessively elevated, the strength of the carbon fiber itself is lowered.
In the process wherein a polymer is attached to the surface of a carbon fiber by electrolytic polymerization, no polymer has been found that can make the interfacial bonding strength between the carbon fiber and the matrix resin sufficiently high, and an industrially or commercially optimum technique has not yet been discovered.
Taking these things into consideration, studies have been made, and a process has been found wherein oxygen and nitrogen functional groups are introduced onto a carbon fiber and electrolytic polymerization is effected.